In today's high speed communication systems, optical components are employed in order to transmit information using optical signals. Usually, optical signals are transmitted over optical fibers, which, unfortunately, distort the transmitted signal due to different transmission channel characteristics at different wavelengths. The distortion may comprise wavelength-specific attenuation or chromatic dispersion, the later resulting when signals components at different wavelengths propagate with different velocities along the optical communication channel.
In order to compensate for the distortion, a digital filter may be employed at the receiver to improve the signal's quality for a subsequent detection of the transmitted information. For digitally filtering the received optical signal, first an optical coherent demodulation and, subsequently, an optical-to-electrical conversion upon the basis of e.g. light sensitive diodes are performed. Finally, an analog-to-digital converter (ADC) provides the digitized signal. Nevertheless, the resulting digital signal still comprises residual distortion, e.g. chromatic dispersion, which can be reduced by way of digital filtering.
For chromatic dispersion filtering, an efficient filter as known from M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, A. Napoli, and B. Lankl, “Adaptive Chromatic Dispersion Equalization for Non-Dispersion Managed Coherent Systems”, OFC 2009, paper OMT1, may be employed. The filter described therein is based upon an error criterion u(t)=|s(t)|2−R which is derived from a complex value time domain signal s(t), where R denotes expectation power. This approach relates to the known constant modulus algorithm (CMA). In order to adapt the filter in the frequency domain, the error signal u(t) is transferred to the frequency domain in order to update the filtering function, i.e. the filter coefficients. After a plurality of consecutive updates, the filter will approximate the ideal filter function Hdis−1(ω) determining the filter coefficients and representing an inverse of the channel filter function introducing chromatic dispersion.